idiomatic "get or else update" for immutable.Map?

scala, scala-collections

Solution

Let me summarise your problem:

- You want to call a method on a immutable data structure

- You want it to return some value and reassign a `var`

- Because the data structure is immutable, you’ll need to

- return a new immutable data structure, or

- do the assignment inside the method, using a supplied closure

So, either your signature has to look like

def getOrElseUpdate(key: K): Tuple2[V, Map[K,V]]
//... use it like
val (v, m2) = getOrElseUpdate(k)
map = m2

or

def getOrElseUpdate(key: K, setter: (Map[K,V]) => Unit): V
//... use it like
val v = getOrElseUpdate(k, map = _)

If you can live with one of these solutions, you could add your own version with an implicit conversion but judging by the signatures alone, i wouldn’t think any of these is in the standard library.

Problem

What is the idiomatic way of a getOrElseUpdate for immutable.Map instances?. I use the snippet below, but it seems verbose and inefficient ``` var map = Map[Key, Value]() def foo(key: Key) = { val value = map.getOrElse(key, new Value) map += key -> value value } ```

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